The energy required to transport natural gas through a pipeline is stored in the form of gas pressure. Therefore as natural gas is transported along a pipeline it undergoes a pressure loss. For any given pipeline and gas flow rate, the magnitude of the pressure loss is primarily dependent upon the pressure and temperature of the gas flowing through the pipeline. The lower the natural gas pressure and/or higher the temperature, the greater the pipeline pressure loss, and vice versa.
In order to maximize natural gas throughput of a pipeline it is necessary to restore the pressure loss (i.e. energy consumed) that occurs as the natural gas travels down a pipeline, by compressing the gas at regular intervals along the pipeline. For any given compressor discharge gas pressure and gas flow rate, the power and thereby energy required to compress the gas is primarily dependent upon the compressor suction gas pressure and temperature (i.e. pipeline outlet gas pressure and temperature). The lower the suction gas pressure and/or higher the suction gas temperature, the greater the amount of power required to compress the natural gas, and vice versa.
However natural gas undergoes an increase in temperature during the compression process. Depending on the length of intervening piping/pipeline and surrounding soil conditions, elevated compressor station discharge gas temperatures (i.e. pipeline inlet gas temperatures) result in lower suction gas pressures and/or higher suction gas temperatures at the downstream compressor station. For any given gas flow rate this results in an increase in the amount of power required to compress the natural gas at the downstream compressor and thereby results in an increase in the amount of energy required to transport the gas through the pipeline.
Air cooled heat exchangers are utilized to lower the temperature of the natural gas to reduce compression power requirements and thereby reduce the amount of energy required to transport natural gas through a pipeline.
In the case of air cooled natural gas heat exchangers, heat transfer occurs between the warm gas and cool ambient air that is forced through the exchanger by powered fans. The amount by which the gas temperature is reduced is dependent upon heat exchanger design (i.e. total cooling surface area, and number, configuration, length and diameter of the cooling tubes, and overall heat transfer coefficient), heat exchanger inlet gas pressure and temperature, ambient air temperature, the specific heat capacity of the natural gas and air, and flow rate of the gas and air through the exchanger.
It is known in the art that the cooling of natural gas is sometimes not energy efficient and consequently, heat exchangers are often shut down and/or bypassed completely to avoid unnecessary energy costs. This invention provides an improvement upon prior art methods of optimizing energy savings during the operation of compressor stations and heat exchangers when transporting natural gas through pipelines.